There are already known various constructions of mirror arrangements, among them such in which the substrate provided with the reflecting surface of the mirror is actively cooled by bringing a liquid cooling medium flow in intimate contact therewith. Actively cooled mirrors of this kind are often used in mirror arrangements for reflecting high energy laser beams, especially because such high energy laser mirrors require high dimensional stability under thermal loading and the actively circulated cooling medium removes heat, which could otherwise distort or damage the mirror or other components of the mirror arrangement, from the mirror.
Standard actively cooled mirror designs usually employ subsurface cooling channels within or in the back of the mirror substrate, and a cooling medium, such as water, is forced to flow through such cooling channels to remove heat which might otherwise build up in the structure. To achieve a useful rate of heat transfer between the mirror substrate and the cooling medium and relatively rapid removal of the thus heated cooling medium from the vicinity of the cooled mirror, the liquid cooling medium is usually forced through the cooling channels at a relatively high speed and at a relatively high pressure relative to the ambient pressure. Typically, effective cooling of an actively cooled high energy laser mirror requires liquid cooling medium flow rates close to twenty gallons per minute. These flow conditions induce undesirable vibrations in and/or pressure related deformations of the cooled mirror or structure. The high pressure and coolant storage volume cause system logistical problems which reduce portability and efficiency. Moreover, as the cooling medium progresses through the respective cooling channel, its temperature rises due to both heat absorption from the mirror substrate and frictional heating. This temperature rise causes an uneven and uncontrollable thermal growth or mapping of the mirror substrate material which changes and degrades the figure or configuration of the reflective surface of the mirror. In addition, the most common coolant, water, is limited by its relatively low boiling point to a lower burnout flux than is often desired. Thus, it may be seen that the heretofore known cooled mirror structures suffer of many inherent drawbacks that, if not insurmountable, are very difficult to overcome.
On the other hand, it is known to use phase change materials to achieve cooling while minimizing the amount of the cooling medium used for such cooling. The so-called heat pipe (i.e. a cooling arrangement that supplies a cooling medium to the component to be cooled in its liquid state for evaporation at the component and subsequent removal of the vaporized cooling medium from the vicinity of the component) has long been successfully used in a variety of terrestrial and outer space applications. Unfortunately, such heat pipe concepts are limited in both performance and thermal flux capacity. Such devices have not yet been practically utilized in high or moderate flux, low distortion applications, such as in high-energy laser (HEL) beam loaded mirrors. This is in part due to dramatic drop in thermal conductivity when the liquid evaporates.
Accordingly, it is a general object of the present invention to avoid the disadvantages of the prior art.
More particularly, it is an object of the present invention to provide an actively cooled mirror arrangement which does not possess the disadvantages of the known arrangements of this kind.
Still another object of the present invention is so to develop the mirror arrangement of the type here under consideration as to improve the heat removal from the mirror substrate while simultaneously reducing or eliminating undesirable thermally and mechanically induced distortions thereof.
It is yet another object of the present invention to devise a mirror arrangement of the above type which improves the uniformity of the temperature distribution throughout the mirror substrate.
A concomitant object of the present invention is to design the actively cooled mirror arrangement of the above type in such a manner as to be relatively simple in construction, inexpensive to manufacture, easy to use, and yet reliable in operation.